Optocouplers are used to electrically isolate an input signal from a corresponding output signal. For example, optocouplers may be used in data access arrangements ("DAAs"). A data access arrangement (DAA) is used for interfacing a data terminal equipment ("DTE") (such as data modems, facsimile machines, non-cellular portable telephones, speaker phones, and message answering machines, for example) with lines of the public switched telephone network ("PSTN"). The network (PSTN) must be protected from potential damage due to, for example, faulty data terminal equipment (DTE) or inadvertent shorts through the data terminal equipment (DTE) to its power line. Indeed, the United States Federal Communications Commission ("FCC") requires a 1500 volt isolation between data terminal equipment (DTE) and the network (PSTN). In the past, data access arrangements (DAAs) have used transformers to provide such electrical isolation. However, due to their relative expense and large size and weight, transformers are disadvantageous, particularly for use in portable data terminal equipment (DTE). Alternative isolation components, such as optical isolators, must be used for such reduced volume/weight applications.
Known optocouplers include an LED which is optically coupleable with, but electrically isolated from, a photodiode. The photodiode ("the output signal photodiode") generates an output signal based on the intensity of light emitted from the LED and detected by it.
Known optocouplers may also include an additional photodiode ("the feedback control signal photodiode") for generating a servo-feedback signal based on the intensity of light emitted from the LED and detected by it. The feedback control signal photodiode allows the optocoupler to operate more linearly. In these known optocouplers, the output signal photodiode and the feedback control signal photodiode are discrete elements. As such, a first direction defined between the LED and the output signal photodiode differs from a second direction defined between the LED and the feedback control signal diode. Unfortunately, the LED may emit directionally non-uniform light. As a result, the intensity of light detected by the output signal diode usually varies from the intensity of light detected by the feedback control signal diode. Consequently, the output of the feedback control signal photodiode will not accurately indicate the intensity of the light from the LED detected by the output signal photodiode, thereby preventing the full compensation of non-linearities in the operation of the optocoupler.
One solution to the problem of directionally non-uniform light emission is to place the output signal photodiode in close proximity to the feedback control signal photodiode. Unfortunately, this offers only a partial solution because directionally non-uniform light will still cause non-linearities, although to a lesser extent. In addition, the feedback control signal photodiode must be adequately isolated from the output signal photodiode to provide adequate electrical isolation. Such isolation is difficult when the two photodiodes are located close to one another.
In view of the above described problems with known optocoupler circuits, an optocoupler immune to directionally non-uniform light emission by the LED is needed. Furthermore, any photodiodes of the optocoupler should be adequately electrically isolated from one another. Moreover, the optocoupler should be relatively simple and economical to manufacture. If possible, the optocoupler should be integrated on a single chip.